Steelmaking processes



4 Sheets-Sheet l May 19, 1970 Filed June 19, 1964 May 19, 1970 l K,BROTZMANN ET AL 3,512,957'

STEELMAKING PROCESSES Fled June 19, 1964 4 Sheets-Sheet :3

May 19, 1970 K. BROTZMANN ET AL 3,512,957

STEELMAKING PROCESSES Filed June 19. 1964 4 Sheets-Sheet 5 Fig. 4

m /O/ Y STEELMAKINC- PROCESSES 4 Sheets-Sheet 4 Filed June 19, 1964 Fig.5

M a 0 ORM. g 0u W90 Q MMU UDHQ mw. AWV JMQVQ n m nu Q 9 Q uM. s a advana o d a 0 0 0 United States Patent() Inf. c1. Calc 7/6, 7/10, 33/00U.s. Cl. 75-49 7 claims To make deep-drawing steels resistant to aging,the steel is de-oxidised with aluminum leaving at least 0.025% of Aldissolved in the steel. Some of the aluminum oxide forming as ade-oxidation product is deposited as large oxide particles near thesurface of the ingot on casting, so that the entire ingot must beflame-descaled. Apart from the cost of this descaling, production isgreatly reduced ias a result. Also, small oxide particles are still leftbeneath the ingot surface and as a result the Al-killed steel cannot beused for purposes requiring best surface qualities. On theoth'er hand,the cold working properties of the steel are so superior to those ofnon-ageing-resistant steel that a considerable amount of deep-drawingsheets 'are produced in this way.

There has been no lack of attempts so to alter the steel structure as toobviate the considerable diiiiculties occurlring during its manufacture.For example, an attempt has been made to render an unkilled steelresistant to ageing by the addition of boron. Since, however, boron hasa certain atiinity to oxygen, the boron greatly reduces the boiling ofthe unkilled steel so that in this case too the resultant steel isimpure. A procedure that has therefore been adopted is to hold up theintroduction of the boron until Va fewV minutes after pouring into theingot, i.e., until a clean outer z'one has already formed. Apart fromthe difficulty of distributing the boron in the ingot, this steel hasbeen unsatisfactory for lack of purity of the ingot. Finally, an attemptwas made to use partially killed steels. Since these steels have a lowoxygen content, it was thought that oxidation of the boron could beprevented.

Apart from insufficient purity, such attempts also resulted inconsiderable surface flaws, due mainly to the peripheral blow holeswhich are inevitable in a semi-killed steel and are situated justbeneath the surface.

The object of the invention is to produce a deep-drawing steel which isresistant to ageing and has properties of Al-killed steel, underconditions which avoid the disadvantages of Al-killed steel and afford avery simple and economic production process.

To this end, according to the invention, the steel is tapped in anunkilled or partiallyA killed state, its free oxygen is largely removed,for example down to a residual contact between 0.005% and 0.010% byweight, by degasification of batches in a vacuum chamber, and the oxygenremaining in the steel is further reduced Iby the addition of aluminumin an amount between 0.001% and 0.005% by weight, after which between0.05 kg. and 0.10 kg. of boron per metric ton of steel is added to thesteel (which is an addition of 0.005 to 0.10% by weight, of boron) andthe steel thus treated is poured rapidly into an ingot mould at a rateof climb in the mould of 80 cm. per minute, preferably 120 cm. perminute.

The process is best carried out using steel with a carbon content ofabout 0.08%, and this steel is tapped into a ladle in an unkilled statefrom an open-hearth furnace and is then vacuum-treated in batches.During this degasitcation process, individual batches weighing about 10%of the ladle content are periodically drawn into the degasiticationchamber from the steel in the ladle through a pipe leading to the bottomof the degasilication chamber and are returned through the same pipe tothe ICC ladle after degasification. The degasication of each batch takesabout l15-20 seconds.

An example of a process in accordance with the invention will now bedescribed with reference to the accompzlnylilng drawings which show theapparatus used and in w 1c FIG. l is a diagrammatic section through theapparatus at one stage of the process;

FIG. 2 is a similar section at a later stage;

FIGS. 3 and 4 are graphs showing the vacuum in the vacuum chamber duringthe process; and

FIG. 5 is a reproduction of an etched section of an ingot made by theprocess.

The apparatus comprises a degasilication chamber 1 having a pipe 2leading to its base, a ladle 3 filled with the melt for degasiiication,a connection 4 from the chamber to a vacuum pump and a tank 5 forintroduction of treatment or alloying substances into the degasicationchamber 1. In the position shown in FIG. l, the pipe 2 dips into thecharge in the ladle 3. In this position a batch of steel S is suckedinto the tank where it is degasified. On raising the vacuum chamber tothe position shown in FIG. 2 the degasified steel ows back through thepipe 2 into the ladle 3. This cycle of batch degasication is continueduntil the steel oxygen content has dropped to the required amount.

FIG. 3 shows the variation of the vacuum with time in the degasificationchamber under these conditions. The abscissa of the diagram denotes timein minutes, and the ordinate vacuum pressure in torr (=mm. Hg). Thevacuum, equal to about 50 torr at the beginning of the degasicationoperation, has dropped to about 5 torr after about 30 batch treatments.With a specifically dimensioned degasilication chamber the pressure risefor a flow of equal batches into the tank is a measure of the freeoxygen content of the steel. The relationship between the pressure riseof the batch entering the degasication vessel and the free oxygencontent of the steel is determined from samples taken from the ladleduring prior treatments.

The result is shown in FIG. 4, which refers to the pressure rise permetric ton of steel taken in. In this diagram the abscissa denotespressure change per quantity in torr/t, and the ordinate denotes freeoxygen content in $5000 by weight. From FIG. 4 it will be apparent thata pressure rise of 0.2 torr per metric ton of steel corresponds to asteel free oxygen content of 0.009% by weight. If it is required todegasify to this value the steel of a 100 metric ton ladle, l0 metrictons of steel being drawn in each batch, the degasication is continueduntil the pressure rise on entry of steelinto the tank is equal to 2torr. The pressure curve depends also on geometric conditions and thecapacity of the suction apparatus used for the degasiiication plant. Theconditions shown in FIG. 4 must therefore be determined empirically foreach plant and each steel grade.

When the free oxygen content of the steel in the degasitication chamberhas dropped to the required value, preferably to between 0.005 and0.010% by weight, aluminium is introduced via the vessel 5 into thebatch in the degasication tank until the free quantity of oxygen in thecharge in the ladle after the discharge of the batch has dropped to0.001-0.005% by weight, preferably 0.003% by weight. The quantity ofaluminium required for the purpose can be calculated correctly becausethere is no incalculable burning away when the aluminium is added viathe discharge vessel. In the case described, the required final oxygencontent in the molten steel was obtained by the addition of grammes ofaluminium per metric ton of Isteel. Boron was then added-again via thedegasiticabatches of steel and return them to the ladle after thealuminium and boron have been added.

A melt prepared in this way was poured into moulds of a capacity of 15metric tons each at a speed such that the moulds were Wull in about 11/2minutes. The ingot head was quenched with water.

FIG. 5 shows the nature of a steel treated in this way. It shows thestructure of an ingot cut open longitudinally. It will be seen that thesteel has no peripheral blow holes otherwise found in half-killedsteels. The core structure of the metal is loosened to a certain extentbut there is no unsatisfactory pipe formation.

The ageing-resistant deep-drawing steel prepared by the method accordingto the invention has no oxide inclusion in and beneath the surface ofthe ingots, such as are found in aluminium-killed steel. The steel cantherefore be rolled very satisfactorily and rough slabbing gives asurface quality such that no ame descaling is required. Anotheradvantage of the steel is the high and uniform yield due to itsfavorable solidication structure. For example, an average yield of 93.5%was obtained for a charge, and varied only between 93 and 94% in theindividual ingots.

I claim:

1. A process for the manufacture of deep-drawing steel which isresistant to ageing comprising the steps of tapping said steel in an atleast partially unkilled state, degasifying said tapped steel in batchesin a vacuum chamber to largely remove the free oxygen therefrom, addingaluminium to said steel to reduce its oxygen content further to between0.001% and 0.005% by weight, adding between 0.05 kg.V and 0.10 kg. ofboron per metric ton of steel to said Isteel and pouring said steelrapidly into an ingot mould at a rate of climb in said mould of at least80 cm. per minute.

2. A process as claimed in claim 1, in which said degasifyng step iscarried out until said free oxygen is reduced to a value of between0.005% and 0.01% by Weight of said steel.

3. A process as claimed in claim 1, in which said pouring step takesplace at a rate of climb in said mould of 120 cm. per minute.

4. The method of producing non-aging steels comprising the steps ofsubjecting a molten steel bath to a vacuum sufficient to cause removalof oxygen from the bath, maintaining the bath under vacuum until theoxygen level in 4 the bath is reduced to about 0.005% to about 0.01%,adding aluminium to the steel in an amount -sucient to react with theremaining oxygen to reduce the oxygen level below about 0.003% andadding 0.005% to 0.010% boron to the molten bath following the aluminumaddition.

5. The method of producing non-aging steels comprising the steps ofsubjecting a molten steel bath to a vacuum sucient to cause removal ofoxygen from the bath, maintaining the bath under vacuum until an oxygenlevel below about 0.01% is attained in the steel, adding an amount ofaluminium to the steel suicient to reduce the oxygen level below about0.003% and adding about 0.005 to 0.010% boron to the molten bathfollowing the aluminium addition.

6. The method of producing non-aging -steels comprising the steps ofsubjecting a molten steel bath to a vacuum sufficient to cause removalof oxygen from the bath, maintaining the bath under vacuum until theoxygen level is reduced to about 0.005% to about 0.010%, adding anamount of aluminium sufficient to react with the remaining oxygen toreduce the oxygen level below about 0.003%, and thereafter adding about0.005 to 0.010% boron to the molten bath.

7. The process of preparing a non-aging steel comprising the steps ofsubjecting a molten bath of steel containing about 0.08% carbon to avacuum suicient to cause removal of oxygen from the bath, maintainingthe bath under a vacuum until the oxygen level in the bath is reduced toabout 0.005% to about 0.01%, adding aluminium to the steel to react withthe remaining oxygen to reduce the level of oxygen below about 0.003%,adding about 0.005% to 0.10% of boron to the molten bath and casting theresulting molten bath.

References Cited UNITED STATES PATENTS 2,768,892 10/ 1956 Shoenberger164-57 X 3,183,078 5/1965 Ohtake et al. 75-49 3,208,844 9/ 1965 Kato etal. 75-49 HENRY W. TARRING II, Primary Examiner U.S. Cl. X.R.

4. THE METHOD OF PRODUCING NON-AGING STEELS COMPRISING THE STEPS OFSUBJECTING A MOLTEN STEEL BATH TO A VACUUM SUFFICIENT TO CAUSE REMOVALOF OXYGEN FROM THE BATH, MAINTAINING THE BATH UNDER VACUUM UNTIL THEOXYGEN LEVEL IN THE BATH IS REDUCED TO ABOUT 0.005% TO ABOUT 0.01%,ADDING ALUMINUM TO THE STEEL IN AN AMOUNT SUFFICIENT TO REACT WITH THEREMAINING OXYGEN TO REDUCE THE OXYGEN LEVEL BELOW ABOUT 0.003% ANDADDING 0.005% TO 0.010% BORON TO THE MOLTEN BATH FOLLOWING THE ALUMINUMADDITION.